Sunday, October 12, 2008

The 7 Stages of Clay

Clay can be classified into seven different stages. So here's your Pottery Vocabulary lesson for the Day.

Stage 1) Raw Clay (Earthenware) -This clay I dug out the side of a hill in the San Rafael during the summer. Clay is easy to spot once you know what to look for. Basically, it's dirt that doesn't have anything growing on it. Utah clay is everywhere; and is lots of different colors (red, purple, black, gray, etc.). This particular clay is a red, low-fire clay.There's also clay that comes pre-mixed, free of air bubbles, and ready to go. this looks something like this, and is what I prefer to use; because it's easy, reasonably priced, and doesn't involve hiking in the desert with a shovel and 50 pounds of dirt to acquire.

Stage 2) Slip - This is Clay that has enough water in it to make it smooth and runny. I use slip to attach handles and decorations to pots, mugs, lids, casseroles and other projects. Slip can also be dried out on canvas and reused. We call this, "ABC Clay", because it's 'Already Been Chewed' once. Here's a picture of a blob of slip sitting out to dry, along with Blob-Man, the temporary class mascot.

Stage 3) Plastic - This is the stage of clay that most of the work is done, like throwing it on the wheel. Plastic clay is soft and easily workable.

Stage 4) Leather Hard - This is the stage of clay that a pot is in when it's half-way dry.Although it's still wet, it is strong enough to support itself, and strong enough to keep it's shape when pressure is put on it. This is the stage where trimming a foot on the pot takes place, handles and decorations are put on, and carving out holes or detail work can take place.

Stage 5) Bone Dry (or Green) -This stage is when the clay is in it's most fragile state; a tiny nudge could potentially knock the lip off of a bowl, or the handle of a mug. The most common destruction I see is when people pick up their pottery by a lip edge or handle and it breaks. Bone Dry clay is when the pot has been exposed to air and all of the water has evaporated out of the clay, and it is left completely dried out. This is the stage in which it is put, very carefully, into the bisque kiln.Stage 6) Bisque - Bisqued clay is clay that has been fired in a kiln, but it is still porous enough to absorb water. In this stage, no additions can be added to the pot, and it is almost complete. This is also the stage that you add glaze to the pot to prepare it for it's final firing.Stage 7) Fired - After your pottery has been Bisqued, it needs to be fired again. There are lots of different types of final firing. Here are the three kinds I use:

High Fire - This is mainly for functional pottery, like dishes and bowls, and is fired at approximately 2300 degrees to make the glaze harden to a hard glass coating. The entire heating and cooling process of High Fire takes about 3 days. The results of this type of fire are usually pretty predictable, unless you're experimenting with glazes like I often do.

Raku - This process is done in a small outdoor kiln, and the pot is heated until it glows red hot (approximately 1800 degrees) and then immediately taken out of the kiln with tongs and either burning horsehair onto it, or placing it into an enclosed area like a garbage can filled with newspaper, pine needles, sawdust, or any other type of combustible material. This process only takes an hour or so, and always comes out different.

Pit Fire - This process involves digging a hole in the ground, placing your pots in it with wood and sawdust, and lighting it on fire. It takes about 5 or so hours to let the pots get hot enough to turn rock hard. Chemicals can be added to the sawdust to make them turn colors, like Iron Oxide, or Cobalt. These pots always come out different, and sometimes need to be fired a couple of times to achieve the type of coloration you desire.

Each firing process results in a completely different product, but either way, from there, your pottery is complete!Until next time,Pinky